The Longitudinal Investigation of Fertility and the Environment (LIFE) Study will follow up to 500 couples who are attempting to become pregnant to determine the relation, if any, between persistent environmental chemicals and five study outcomes: 1) time to pregnancy;2) infertility;3) miscarriage;4) gestation;and 5) infant birth weight. A prospective cohort study of couples who are interested in becoming pregnant and who live in two well-defined geographic areas is capturing longitudinally data on lifestyle factors and menstrual cycle characteristics during the period in which the couples are attempting pregnancy. Couples will be recruited prior to conception to assess a spectrum of sensitive reproductive and developmental outcomes that can only be ascertained in this manner (e.g., time required for conception and early pregnancy loss as measured by human chorionic gonadotropin hormone). This study will allow us to capture most postimplantation pregnancies and losses, long before clinical recognition, to assess the effect of periconceptional environmental exposures on human conception, gestation and development. The LIFE Study is the first prospective epidemiologic cohort study with longitudinal capture of data that will identify female, male, and couple-mediated determinants of human reproduction and development. The effect(s) of persistent environmental chemicals on human reproduction will be analyzed in the context of other lifestyle factors that may either act to adversely affect human reproduction or to ameliorate the effects of chemicals.